Don't do this!The 150 pf capacitor isn't a "bypass". It's part of the bridge neutralizing circuit. Do not increase its value!The cathode of the 6146 is grounded for RF by its bypasses.73 de Jim, N2EY

interesting ! I saw the evolution of the posted circuit in the 1975 arrl hb (pg. 172) and there's a 1500 pF in place of the 150 pF and is going to ground instead of going to the cathode of the 6146 (as shown in the 3rd image posted above)...

If you don't neutralise the 6146B, that's how you do it. Neutralising a 6146B isn't strictly necessary below 30MHz in CW, although it is for AB1 SSB use, especially on the higher HF bands. The DX40 didn't neutralise the 6146 and that was fine. Some positive feedback (but NOT enough to allow it to oscillate!) can help reduce the drive requirements, and that can help especially on 10 metres.

The DX-40 and DX-35, and some other rigs, used a design trick to avoid neutralization: Low driver impedance.

There is a rather complex formula which defines the criteria necessary for a tuned-grid-tuned-plate amplifier to oscillate. The tendency to oscillate increases with increased Cgp, increased gain, increased frequency, and some other factors. And one of the factors is the impedance of the grid circuit.

What the DX-40 and DX-35 do is to use a pi-network between the driver plate and final grid. The output Z of the pi net is quite low, which results in an intentional mismatch and low efficiency. But since the driver tube provides much more RF than the final needs, the low efficiency isn't a problem.

The 6146 in the DX-40 and DX-35 tis therefore stable without neutralization because of the trick.

Other AM rigs not using particularly low Z drivers that didn't neutralise the 6146 were the UK ones - Panda Explorer, KW Valiant and KW Vanguard, and Labgear LG50. Very few UK rigs used neutralisation in the AM days! Maritime transmitters with three 807s didn't either, even on 22 MHz.

Jim, I think the stability criteria you're thinking of is, for other than a Class C stage

gm.2.pi.f.Rg.Rp.Cpg must be less than 2.

gm is the tube transconductance, f is frequency, Rg is the dynamic resistance in the grid circuit, Rp is the plate load and Cpg is the plate to grid capacity.

With a straight tuned grid circuit, Rg will be QXL, shunted by the plate resistance of the driver, and in Class C, will be reduced further by the grid current.

Thank you everyone for your replies! I’m still working on debugging this transmitter. For a 1965 vintage design, this transmitter is making me feel quite dumb! Following are some findings to date. I think I’ve responded to all the queries? (I think I may be experiencing multiple issues here.)

G8HQP – The loading seems to be working from the following observation .. I can “peak” the loading capacitor C4. The plate current peaks in the middle of C4’s travel but I can only get “close” to 3.4 mA plate reading .. Sometimes I can get 3.0 mA or so (representing about 150 mA of plate current v.s. 170 mA suggested.)

N3QE – When I connect the 6AG7 cathode to ground, the transmitter “fires” every time I key down the 6146B stage. So, I agree that the oscillator is at least part of my problem. (Of course, my receiver/monitor can "hear" the oscillator running perpetually.)

G3RZP – I get the same effect with other crystals, but the transmitter seems better behaved at higher frequency (i.e., 40m as compared to 80m). I don’t have another 6AG7 to try right now. Yes, grounding the 6AG7 cathode “fixes” the oscillator problem. BUT .. I have discovered another issue by doing this. Now, the output section appears to break into some sort of secondary oscillation .. maybe a parasitic when sweeping the Plate and Load capacitors. ALSO .. I have discovered that the amplifier section (6146B) can be made to oscillate WITH THE 6CA7 completely removed! This must be input/output circuit feedback or some neutralization failure?

K8AXW – I think the metering is fairly close. I’ve replaced the 10 Ohm and 470 Ohm resistors for the plate circuit with 1% resistors. If I set the Plate current to 2.3 mA indicated by the Weston 301 panel meter and measure across the 470 Ohm resistor with a Fluke 8010A, I read 1.105 VDC. ( 1.105/470) = 2.34 mA calculated and the meter displays 2.3 mA which is close. If I put my Fluke 8010A in series with the Weston 301 meter, the current measurements as close: Weston 301 = 2.5mA, Fluke 8010A = 2.59mA. Likewise, if I measure across the 27K Ohm resistor in the Grid circuit (26.5K Ohms measured) I read 71.9 VDC. (71.9/26.5) = 2.71 calculated, Weston 301 2.5mA displayed.

KC2ZFA – I have not tried this yet. But I see what you mean. They’ve also added a 100 Ohm resistor in series with the 6146B grid.

N2EY – 1.) I think the meter is fairly close now (see comments to K8AXW). 2.) When in 80m range, peaking C1 will drive the meter off range as the ARRL article says it “probably” will. They suggest de-tuning C1 to bring the drive down to 2.5mA. That is what I have to do. 3.) I am using 0B3 and 0C3 as indicated in the ARRL schematic. Voltages are as follows:B+ 832VDC Key Up, 750VDC OP/KEYDOWNScreen 197VDC Key Up, 193VDC OP/KEYDOWN (in CW/125 Watt mode)Screen 109VDC Key Up, 108VDC OP/KEYDOWN (in PH/75 Watt mode)

I'm including some links to some photographs. Maybe my layout is bad? Maybe the oscillator is too close to the 6146 and some shieldingis needed? I'm using a "temporary" front panel.

I would maximise the drive control. I dont expect oscillator will start cleanly at low screen voltages, so check it keys perfectly to start with. (With the 6146 removed)

The PA looks like a tuned-plate-tuned-grid oscillator to me and would be bound to burst into violent oscillation when you approached resonance. I would take out or minimise C2, change the 150pf to .01uf. Connect 1K to 10K 2W across C1 and try tuning up again, on 40 metres. That might dampen the circuit enough to get it working on fundamental xtal freqs, but not when you try to double.

Hope your 6146 is still OK. I had serveral go blue and fail in an old tx I bought recently.BRAndrew

Something else I dont like about the circuit. The key line is going to rise towards full HT on key up. I would try connecting something like 4K7 3W across the key to reduce the voltage until the PA just had some small current like 20 mA on key up. 73Andrew

The PA looks like a tuned-plate-tuned-grid oscillator to me and would be bound to burst into violent oscillation when you approached resonance. I would take out or minimise C2, change the 150pf to .01uf. Connect 1K to 10K 2W across C1 and try tuning up again, on 40 metres. That might dampen the circuit enough to get it working on fundamental xtal freqs, but not when you try to double.

The 6146B stage is neutralized by the combination of C2 and the 150 pf capacitor! Doing what you suggest removes the neutralization and will probably make the transmitter more unstable.

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